This invention relates to electronic flat panel displays and, more particularly, to a method for assembling tiled semiconductor microdisplays into a single composite flat-panel display, the projected image of which has visually imperceptible seams.
Microdisplays (xcexcDs) are the most recent addition to the family of flat-panel displays. While xcexcDs are based on a number of different techniques to generate or modulate light, all use microfabrication to produce a rectangular array of pixels on a semiconductor back plane, usually silicon. Examples of demonstrated xcexcDs include liquid crystal displays (LCDs), field emission displays (FEDs), and digital micro-mirror displays (DMDs). Pixels in xcexcDs can be fabricated to have pitches in the range of approximately 10 xcexcmxc3x9710 xcexcm to 30 xcexcmxc3x9730 xcexcm. For a 10 xcexcm pixel pitch, the display array size is approximately 10.24 mmxc3x977.68 mm assuming XGA resolution (1024xc3x97768 pixels). Control, driver and image processing circuits may be embedded into the back plane. When viewed through a suitable magnifying lens, such xcexcD pixel arrays can be designed to appear to the human observer as equivalent to a desktop monitor (e.g., a 15xe2x80x3 diagonal monitor) when viewed at a distance of approximately twice the diagonal dimension. In such magnified applications, xcexcDs are suitable for use, for example, in portable television (TV), compact disc (CD), digital video disc (DVD), personal digital assistant (PDA) applications, and the like.
In contrast, when projected on the front or back side of a large screen through an optical system, xcexcDs have the potential to produce images that rival conventional projectors using polycrystalline LCDs. In this projection configuration, xcexcDs may be used in applications such as large screen TV, multi-user (multi-viewer) computer, multi-media, and home theater. However, many design and fabrication problems have heretofore prevented the realization of this potential. For example, reflective microdisplays generally have a rather low geometric efficiency, which makes high brightness projected images difficult to achieve. Microdisplays also force the use of small size arc lamps or special lamps that provide high luminous output at small source size in order to maintain the geometric efficiency. High resolution xcexcDs with larger pixel pitches also would require unacceptably large chip sizes.
The construction of a typical reflective LCD xcexcD device is described hereinafter and serves as a starting point for describing tiled xcexcD assemblies. The tiling structures, fabrication methods, and circuits for other reflective or emissive xcexcDs are essentially the same and are not described separately.
The back plane of a typical xcexcD is formed from a crystalline silicon chip which includes integrated circuits (ICs). Typically these are of the CMOS family. Therefore CMOS is used here to represent all other suitable integrated circuit families. The CMOS ICs used in xcexcDs are fabricated using a typical SRAM-like process as is well known to those skilled in the art. Minimum feature sizes of less than 1 xcexcm are typical on such chips. No significant difference in the fabrication process compared to standard CMOS chips occurs until the application of the upper levels of metal interconnect. Multi-layer Al/SiO2 metalization is still used but the topmost metal layer forms a two-dimensional array of rectangular mirrors, each about 10-30 xcexcm on the side with a gap of about 1 xcexcm between each pair. These mirror elements serve as the pixels of the xcexcD; the topmost Al layer being polished to a mirror finish in order to serve as a highly effective optical mirror with a reflectivity generally greater than 80%.
The gaps between the mirror elements are filled with a dielectric material, typically SiO2, with low optical reflectivity. The ratio of the optically active area of each mirror to the entire area of the mirror plus any optically inactive areas, such as the gaps, is called the aperture ratio. Typical xcexcD aperture ratios are on the order of 85%, much higher than is possible with direct view transmissive active matrix liquid crystal displays (AMLCDs). The metal layer immediately under the mirrors forms light shields under the gaps that prevent light from reaching the light sensitive CMOS circuitry in the back plane.
Each mirror element is connected to a CMOS driver circuit through single or multiple vias that provide the voltage to that particular pixel. The rest of the metal interconnect in/on the back plane is used for conventional addressing of the pixels (e.g., matrix addressing) and for regular circuit functions and services for the CMOS circuitry. The CMOS back plane can also contain some or all of the circuits needed for display addressing (e.g., row and column drivers for matrix addressing), control circuits, and any desired image processing circuits.
Given the CMOS back plane with the mirror elements, the xcexcD is assembled as follows. A passivation layer and an optional LCD alignment layer are applied to the top of the mirror plane. A seal bead with a small fill port is next dispensed around the pixel array in the periphery using screen-printing or a dedicated dispensing system. Separately, a glass cover plate is fabricated having on its lower side a conductive transparent electrode film (e.g., indium-tin-oxide (ITO)), and possibly another alignment layer for the LC material. Large area microfabrication techniques may be used to make arrays of cover plates, which may be scribed and broken into appropriate sizes.
The cover plate is placed on the seal, aligned to the CMOS back plane mirror array and then bonded to the seal bead. The display is next filled with a suitable liquid crystal (LC) material, such as a twisted nematic liquid crystal (TN-LC) or a ferroelectric liquid crystal (FLC) material, and then the fill port is sealed. This LC fill may also contain spacer particles that are dispensed throughout the fill, unless spacers have optionally already been fabricated on top of the mirror array or cover plate using microfabrication techniques. Electrical connections from the conductive electrode on the underside of the common cover plate to the silicon back plane are made at the same time, for example using conductive adhesive.
A polarizer film may be applied on the top surface of the cover plate or placed elsewhere into the optical system. Next the xcexcD component is mounted on an interconnect substrate, usually flex, and electrical connections are made from the edge of the CMOS back plane to the substrate. Finally, the xcexcD component is suitably encapsulated, thus providing environmental protection. Plastic encapsulation is typically used in consumer products. The resulting xcexcD modules produced in this manner are compact, lightweight, and relatively inexpensive.
The optical systems for use with xcexcDs provide three separate functions: (a) provision of light, (b) formation of color, and (c) magnification of the image to the desired size. There are several ways to produce color. Most direct view transmissive AMLCDs form color by placing a color triad (e.g., red, green, and blue) into each pixel, using white back light and patterned color filters. This is called the spatial color generation technique. Since this increases the pixel pitch by a factor of three compared to a monochromatic pixel array, this technique is not usually preferred in xcexcDs. The second way for producing color is to use a separate display unit for each color and then to combine the colors into a single, final image. This so-called three channel approach is the favored technique in commercial front-projection displays with polysilicon transmissive LCDs. However in xcexcD applications, this approach may be acceptable only in large rear projection systems. In the third method, the mirror array is illuminated sequentially with the different primary colors, one at the time, thus forming the proper color mix as a time average in the human vision system. This is called the field sequential approach. The sequential field colors can be formed from white light by using a three-color filter on a xe2x80x9ccolor wheelxe2x80x9d or from three separate color light sources. In some magnified view xcexcDs, compound semiconductor solid state light emitting diodes (LEDs) are modulated to produce the field sequential illumination of the mirror array. Field sequential operation requires a pixel response time that is fast enough to resolve the short illumination times. For example, at VGA resolution and a frame rate of 60 Hz, the pixel response time should be on the order of 30 xcexcs or better.
The magnification of the image can be accomplished using refractive or reflective lens assemblies that are well known and widely utilized in standard optical projection systems.
Consider as an illustration the characteristics of a xcexcD manufactured by Displaytech of Longmont, Colorado. Their VGA display has 640xc3x97480 pixels, a pixel pitch of 13 xcexcm, a pixel spacing of 1 xcexcm, FLC fill, an aperture ratio of 85%, an 85% mirror reflectance, field sequential 15 bit color (32,768 colors), a pixel array size of 8.32 mmxc3x976.24 mm, and a 60 mW chip power consumption. The full display engine uses field sequential illumination from red, blue, and green LEDs, and a single external polarizer. The pixel response time is on the order of 30 xcexcs, fast enough to support field sequential operation.
Although currently available xcexcDs provide only VGA and SVGA resolutions (640xc3x97480 and 800xc3x97600, respectively), much higher resolution devices are anticipated in the future. The following table summarizes characteristics of higher resolution xcexcDs that may be available in the future. Two pixel pitches, 10 and 30 xcexcm, are given in this table. The 10 xcexcm pitch is representative of field sequential or multi-channel color and the 30 xcexcm pitch of spatial color. The right-most column specifies the magnification factor for rear projector applications with a 40xe2x80x3 screen diagonal. Similar numbers can be generated for magnified view xcexcD applications. It can be seen from this table that higher resolution displays lead to very large chip sizes, especially for spatial color modulation. Such chip sizes are impractical from the point of view of the manufacturing yield and cost.
It is therefore an object of the invention to provide methods for fabricating tiled, flat-panel displays composed of multiple microdisplays (xcexcDs) in order to overcome the size limitation of higher resolution monolithic displays.
It is a further object of the invention to provide a method for fabricating a tiled, flat-panel display composed of multiple microdisplays and having visually imperceptible seams between the xcexcD tiles.
It is an additional object of the invention to provide a method for fabricating tiled, flat-panel xcexcD displays having the xcexcD tiles attached to a common substrate.
It is another object of the invention to provide a method for fabricating tiled, flat-panel xcexcD displays having common semiconductor substrates.
It is a still further object of the invention to embed control circuity in either the semiconductor back plane of the xcexcDs and/or in the common semiconductor substrate.
It is an additional object of the invention to fabricate a common substrate which is thermally and mechanically matched to the thermal and mechanical characteristics of the individual xcexcD tiles.
It is yet another object of the invention to fabricate cooling structures as part of the tiled, flat-panel xcexcD structure to maintain the operating temperature of the display.
It is a still further object of the invention to fabricate heat-generating means in the display which, when coupled to appropriate temperature sensors and control circuitry, can dynamically maintain essentially a fixed temperature in the tiled, flat-panel xcexcD assembly.
The present invention describes methods for fabricating larger size microdisplays through a technique called physical tiling, in which multiple microdisplay (xcexcD) components (tiles) are assembled together such that the resulting composite display appears to the observer as a single, monolithic display with no visually perceptible seams, discontinuities, or non-uniformities. The objective is to overcome the chip size, geometric efficiency, resolution, contrast, yield, and back plane technology limits inherent in monolithically fabricated microdisplays. The lower magnification facilitated by the larger pixel array size simplifies projection and/or eyepiece optics and reduces aberrations and chromatic lens distortions.
The display tiles in the preferred embodiment are arranged in a regular two-dimensional array such that all tiles have at least one external edge and one to three internal edges, thus allowing the connections to each display tile to be made at one or more external edges. This leads to 1xc3x972, 2xc3x971, 2xc3x972, 2xc3x97n and nxc3x972 tile arrays. Larger nxc3x97m arrays may be constructed with connections brought to the top surface of the xcexcD tiles by deposition of metal and circuitization on the edges of the xcexcD tiles. The xcexcD tiles can be fabricated on silicon CMOS wafers using the same or similar microfabrication techniques that are used for monolithic xcexcD chips. Tiles may also come from different fabrication lines or chip technologies. The internal edges are fabricated using lithographic pattern definition and special die separation techniques to a high precision on the order of 1-10 xcexcm or less. The required space for the internal seams are provided by special layout of the mirrors on the xcexcD tiles. The thickness and planarity of the tiles is controlled using chemical-mechanical polishing (CMP). The tiles are then assembled into a precise nxc3x97m array on a thermally matched substrate, the seams between the tiles are filled with a suitable sealant and, optionally, spacers may be provided. The fabrication of the composite tiled xcexcD then follows the conventional process used for making monolithic xcexcDs.
The optical magnification and projection system for the composite xcexcD can be designed and fabricated in the same fashion as that for conventional monolithic xcexcDs, except for the fact that the field size can be nxc3x97m times larger than that of a single xcexcD. These composite xcexcD arrays provide a larger viewable area for magnified displays, thus increasing the number of pixels and the achievable resolution, improving the geometric efficiency, enhancing the viewing angles, and reducing eye strain. Similarly, tiled projection xcexcDs can provide larger pixel counts, higher resolutions, larger image sizes at the same total magnification, better optical efficiency, lower optical power density, and higher contrast for the same viewable image size.